Chrome provides a UI for configuring extension-defined shortcuts, but I don't believe that exists yet in Firefox. Not sure if that's planned or if it's possible to do so via about:config. (Before Firefox 57 you could use the 'keyconfig' extension to adjust all shortcuts.) Since configuring shortcuts is something that applies to all extensions and should be built-in functionality (the setting in Chrome and Firefox is even called "suggested_key"), I don't think it's something we'd offer a preference for.

On macOS you can override any menu option in any program from the system preferences (which would allow you to change the Debugger shortcut). I don't know if there's an equivalent on Windows.

If there are some existing settings in Firefox (like keyboard shorcuts), I think it's problematic if Zotero created some conflicts. Is there any solution for modify Zotero keyboard shortcuts? Or this isn't possible for the same issue you pointed?

I mean, there are only so many keys available, and the dev tools claim a massive number of keys. Zotero for Firefox has been using Ctrl/Cmd-Shift-S for saving since 2013, so this isn't new in the Zotero Connector.

And you're actually mistaken here. Ctrl-Shift-S is just for the Debugger pane. You can open the toolbox to the last-used tool with Ctrl-Shift-I (or use any of the other keys on that page).

(This also doesn't conflict at all on macOS, where the devtools use Cmd-Option and Zotero uses Cmd-Shift.)

Is there any solution for modify Zotero keyboard shortcuts?

Again, this is a UI Firefox should (and will) provide, as Chrome already does.